Represent reflection template arguments with `TemplateArgument::StructuralValue`

[katzdm]

This project first began shortly before the introduction of TemplateArgument::StructuralValue, but Corentin Jabot kindly pointed out that we can probably replace TemplateArgument::Reflection with TemplateArgument::StructuralValue.

[katzdm]

One thing to note here is that a template instantiation that's been specialized by a reflection may be forced to have internal linkage depending on the entity reflected by its template argument. Investigation will be needed as to whether that characteristic warrants a special template argument kind.